Using Polyurethane to Reduce the Production Cost of Hydrokinetic
Turbine Foils
David Norta
1,2, Christoph Lanser
2, Jürgen Sachau
1, H.-J. Allelein
21
Université du Luxembourg, Luxembourg,
2RWTH Aachen University, Germany
ABSTRACT
Polyurethane made profiles can be used for hydrokinetic oscillating foil micro turbines to reduce their production cost. For this purpose, a NACA 0015 profile is designed with a computer aided design software and milled from aluminium. From this sample a negative mould of silicone is produced. This negative was used to produce finally polyurethane foils. Holes and grooves can be simply implemented in the negative by placing corresponding shapes made of metal within the silicone form. A massive reduction of the production cost of about 1600 Euro for a polyurethane (PUR) foil of 1000 mm length compared to an aluminium foil of the same shape is accomplished.
INTRODUCTION
Hydrokinetic turbines are currently living an increased worldwide interest. Their small size, their simple and fast installation, as well as their low impact on the nature are some advantages of the technology. In this publication hydrokinetic river turbines are considered, those turbines have normally a smaller size, harnessing the shallow rivers current, compared to the larger turbines installed close to the seashores. The rivers potential is largely unexplored, whereas coastal test turbines are already under operation [1,2]. Especially smaller rivers are often not explored, due to the small size and the varying flow conditions. Some attempts are made to develop small hydrokinetic turbines to use the untouched kinetic energy of the worldwide creeks. In this publication we want to consider a new type of vertically oscillating hydrofoil turbine, which is developed at the Université du Luxembourg in cooperation with the RWTH Aachen University, Germany [3,4]. In the prototype a NACA 00015 profile is moved by the force of the rivers current vertically, normal to the flow direction through the river. The foils position is controlled by two chains connected independently to two generators, which change the velocity of two rods, one connected moveable in one direction in a longitudal groove along the chord of the foil, the other in a hole in front of the groove both on top of the foil. Due to the small size of the foil of 1000mm x 240 mm x 36 mm (depth x length x width) and the flow velocities hardly reaching 2,5 m/s a polyurethane foil has the sufficient strength to withstand the hydromechanic forces. The rated power of the connected machines does not exceed 9kW and 265 Nm per machine.
Turbine Concept
Figure 1. Schematic drawing from the top view of the horizontal oscillating hydrofoil
turbine.
As seen in figure one, guide rails keep the foil on a normal trajectory relative to the floating fluid. Two chains control the foils angle relative to the flow direction. Two generators moving the chain wheels on the middle pontoon. One generator controls a pair of chains on its left and right side. The outer pontoons carry the corresponding chain wheels to the generator controlled ones. With this setup we ensure that the foils are moved in a controlled motion normal to the water flow.
Figure two shows the turbine concept in three dimensions, compared to figure one the middle pontoon is missing, the whole control mechanism is carried by a frame connected to a lifting mechanism connected to the outer pontoons. The blue line indicates the water level. In figure four the lab setup is shown. Compared to the two conceptual drawings seen before, just one foil is moved. On the left and right side of figure four the controlling generators are shown, indicated by the numbers one. Number two indicates the grid connecting cable. Number three shows the guide rails. Number four are the outer chain wheels, number 5 is the schematic drawing of the hydrofoil and number 6 and 7 are mentioned to represent the generators controlling inverters.
Figure 3. Laboratory test setup of a single foil, hydrokinetic turbine with 1-generators,
2-grid connection, 3-guide rails, 4-chain wheel bearings, 5-hydrofoil, 6/7-inverter and
control not in the picture.
Understanding the principle of a horizontally oscillating hydrofoil turbine it can be easily seen that by lifting and lowering the whole turbine mechanism outside the water the systems immersion depths can be varied. Following the idea of variable immersion depths turbines, shallow rivers can become potential new hydropower sites.
MOTIVATION
A low cost per kWh generated by a new generation technology is crucial in order to become economically competitive. Due to the intended small system size of the novel hydrokinetic turbine, the main competitor are solar photovoltaic systems as well as small wind turbines in the range of 15-20 kW. For a the first prototype with two 1000 mm hydrofoils a price of 23300 Euro is estimated, where the two aluminium hydrofoils have a share of 18 percent. By changing the hydrofoils material and production process from milled aluminium to moulded PUR, their cost can be significantly reduced, while still meeting the stress requirements of the foils material.
the guide vanes and their operation for 7600 hours without any complication the same material was chosen for our prototype.
METHODOLOGY
In this paper the process of producing hydrofoils for an innovative micro hydrokinetic oscillating foil turbine is described. In the following each of the different steps needed on the way to produce a plastic hydrofoil are described. Based on the production of the aluminium sample form, a silicone form is casted. Afterwards, a first PUR test foil is produced in the silicone form. In a second step the silicone form is modified with inlays so that the PUR foil contains a hole for the guiding rod. In a later step also a part is included in the negative to directly keep the space for the groove in the PUR foil.
Having produced the plastic hydrofoil, its strength is calculated in a FEM simulation. Finally the price difference between hydrofoil and PUR foil is calculated. The paper finishes with an analysis of the recyclability of the proposed PUR foil.
RESULTS AND ANALSYSIS
Production of PUR foils
In a first step an aluminium NACA 0015 profile of the following dimensions was milled from an aluminium block 200 mm x 240 mm x 36 mm (height x chord length x width). This block is used to cast a negative form. It was also used to estimate the production cost of such a profile part.
Figure 4. Aluminium hydrofoil 200mm x 240 mm (height x length).
Figure 5.Plan of the box which contains the silicone form from the aluminium foil.
Figure 6. Box manufactured from wood with aluminium foil inside.
Figure 7. Box filled by silicone to produce the negative silicone form for moulding the
PUR foils.
Once the silicone form is hardened it has a Shore hardness of A-42 and is transparent. The box is opened and due to the flexible material the aluminium foil is easily removed. Afterwards the two component polyurethane Biresin G26 is used to produce the positive. The two components resin, component “A” and curing agent, component “B” are mixed in the mass fractions 1:1 (A:B). It has to be ensured that the material is mixed within short time, since the crosslinking starts after 3 to 4 minutes. After 30 minutes the form is stable, the final hardness is reached after three days. Before filling the silicone form with the two components a release agent is distributed on it, for an easier removal of the hardened foil.
After a first trial of producing simple profiles the negative form has been completed by adding a rod for the final connection of the turbine prototype and the foil. The included rod in the casting form avoids a later drilling of a hole, since the liquid PUR wraps the rod and a hole is in the foil once it is removed from the form. All additional inlays have to be covered as well entirely by the liquid release agent to simplify the removal of the casted foil from the silicone form and its inlays.
Forces and FEM Analysis
To ensure that the produced foil withstands the hydromechanic forces while operation of the prototype on the test site, a finite element method (FEM) simulation was used to calculate the forces within the material. We assumed a constant contact force over the length of the rods on the foil. The forces were estimated for the prototype for the maximum moments of the machine and the related chain forces on the rod within the foil. A maximum force of 15000 N for the 9 kW Baumüller DSD2-100BO64U-12-54 machine is possible. This would lead to an average water pressure of 0,061 MPA on the surface of the foil. 2D Computational Fluid Dynamics simulations estimated maximum normal force for a foil of one lower magnitude. The maximum values reached in the FEM simulation for the PUR are:
Table 1: Results of the FEM Analysis of PUR.
Parameter 0,061 MPa 1. Main tension [MPa] max. 12,425 1. Main tension [MPa] min. -14,175 2. Main tension [MPa] max. 9,144 3. Main tension [MPa] min. -16,237
The specifications of the PUR are the following
Table 1. Specification oft he used PUR.
Parameter PUR G26
Density 1,075 g/cm³
Yield Strength 25 MPa
Tensile Strength 30 MPa
Modulus of Elasticity 1,25 GPa
PoissonNumber 0,48
Comparing the two tables it can be seen that neither Yield Strength nor Tensile strength of the material are reached. The maximum stress is given for the third main tension with -16,237 mPa which is about 64% of the materials yield strength.
Economic Analysis
The financial analysis shows that the application of PUR foils has several financial advantages compared to the usage of aluminium. Generally, the cost for the production of an aluminium foil can be split in the following parts. The material and the milling of the foils. The cost of the used machines to drill the hole and to mill the groove. The staff costs for the production of hole and groove. Due to the limited length of the milling heads a 1000 mm foil consists of five 200 mm part-foils.
Table 2. Production Cost in Euro, including the different manufacturing steps for five
200 mm aluminium foils.
Material and milling of 5 foils 1254 Machine cost for hole and groove 20 Staff cost for hole and groove (first foil) 191,25 Staff cost for hole and groove (after first foil) 170
Entire Cost 2145,25
Producing a moulding form from silicone and afterwards the PUR foils from it, including the hole and the groove we come to the following cost distribution and a simpler manufacturing process.
Table 3. Production Cost in Euro including the different manufacturing steps for five
200mm PUR foils.
Material cost silicone 244,9 Material cost PUR 89,6 Material cost Casting frame 50 Material cost separating wax 13,9 Material cost Silicone spray 6,45 Material cost rod and groove-part 20 Staff cost rod and groove-part 42,5
Entire Cost 467,35
Comparing the two foils a cost reduction of 78% is reached. Assuming two foils per turbine the entire cost of the turbine can be reduced by 14% down to about 20000 Euro from former 23300 Euro. Since the modular setup of the turbine allows the usage of several hydrofoils the significant cost reduction of the foils contributes in a mayor way to the cost efficiency of the novel turbine.
Additionally the weight of a full foil is reduced form a nearly 16 kg aluminium foil to an about 6 kg PUR foil.
Recycling
nitrogen-containing products, such as polyurethanes, but also other materials of natural and synthetic origin, volatile nitrogen compounds such as ammonia, nitrogen oxides, nitriles and, at temperatures of 800 - 1000 °C, hydrogen cyanide are also formed. As the predominant risk factor in the fumes of all organic materials CO must be considered. With the used polyurethane material no negative effects can be expected and the used amount for the water turbines entire foil mass is with a mass of 6 kg way less to have any negative consequences by releasing CO.
Besides the low risk of environmental pollution, PUR also has the advantage of easy recycling. This can be separated into the following categories, each of them offering different kinds of methods to handle the PUR: Mechanical recycling Adhesive pressing Particle bonding Regrind Injection moulding Compression moulding Feedstock recycling Glycolysis Hydrolysis Pyrolysis Hydrogenation Gas production
The most common mechanical recycling method is called “regrinding”. Here the PUR is first granulated and then grinded in various ways to a fine powder. This is added afterwards to the production of other PUR products, mostly to the liquid reactant or also used in reaction injection moulding (RIM), see figure 9. Moreover the granulated particles can be used together with other recycled polyurethanes for particle bonding. The granulated PUR gets blended with a binder and then shaped into boards, car bumpers, benches, refrigerator parts and other moulding with high recycled content under high pressure.
Figure 9. RIM recycling of PUR granulate [7].
Overall the most widely used chemical recycling method for PUR is Glycolysis to recover the polyols for production of new PUR materials. Used PUR from industrial and private used gets mixed with diol under high heat, which is causing a reaction creating new polyols. This can later be used to produce new polyurethanes for all kind of applications.
During the hydrolysis the reaction between water and PUR is creating polyols, which can be used as fuel. Moreover intermediate chemicals are a by-product of the reaction and are used as raw materials for PUR.
With help of the Pyrolysis the PUR breaks down and results in oils and gas. By use of hydrogenation, hydrogen, heat and pressure lead to the creation of the same products.
CONCLUSION
ACKNOWLEDGEMENT
This work is conducted as a part of the project “Integration of distributed controllable renewable generators in the Luxembourgish electricity system including innovative micro-hydrokinetic turbines” funded by the Fonds National de la Recherche, Luxembourg in form of a personal scholarship for D. Norta.
REFERENCES
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[2] B. White, “Marine and Hydrokinetic U.S. Resource Assessments and Technologies”, U.S.
Department of Energy, USA, 2012
[3] D. Norta, J. Sachau, H.-J. Allelein, “Modular Hydrokinetic Micro Generation”, Hydro 2013
Conference, Austria, 2013
[4] D. Norta, J. Sachau, H.-J. Allelein, “Horizontally Oscillating Hydrofoil-River-Turbine with
Variable Immersion Depth”, Hidroenergia 2014 Conference, Turkey, 2014
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[7] R. Zevenhoven,“Treatment and Disposal of Polyurethane Wastes: Options for Recovery
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2004
David Norta obtained his Mechanical Engineering diploma in the field of turbulent flows analysis from the Technical University of Aachen in November 2012. He already worked in several international energy projects in India and Fiji in the frame of Engineers Without Borders Germany and started Engineers Without Borders Luxembourg. His PhD thesis title is, “Integration of distributed controllable renewable generators in the Luxembourgish electricity system including innovative micro-hydrokinetic turbines”. David’s interests of research area are renewable energies, micro-hydrokinetic turbines and
smart grids.
Christoph Lanser was born in Aachen, Germany, in 1987 and started studying mechanical engineering at the RWTH Aachen after completing his high school diploma. His studies are specialized in plastic engineering and he has been working as an assistant scientist at the Institute for Combustion Technology since 2011. There he has been running experiments in the field of biofuels for the last two years. Christoph Lanser’s interests include traveling around in the world, learning about diverse cultures and sharing innovative ideas with others interested in developing ways for improving society as a whole.
Jürgen Sachau is professor at the University of Luxembourg in the field of electrical engineering he is the coordinator of the “Reliable Decentral Energy Systems” group and supervisor of Mr. Nortas PhD thesis.
